IB/core: Remove unnecessary check in ibnl_rcv_msg

RDMA_NL_GET_OP is defined like this: (type & ((1 << 10) - 1))
which means op (defined as an int) can never be a negative number.

Fixes: b2cbae2c

 ('RDMA: Add netlink infrastructure')
Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>